Optical image sensors are commonly used in a wide array of electronic devices. These image sensors can vary in size, in features, or in technology, as well as other characteristics, and can be configured to be suitable for various electronic applications. Such applications can include, for instance, digital cameras, video cameras and camcorders, camera modules for laptop computers, flat-screen monitors or portable handheld devices (e.g., mobile phones, personal digital assistants (PDAs), . . . ), conspicuous or inconspicuous video surveillance equipment, and so on. As digital cameras and video cameras become more popular with the consuming public, demands for additional features and more powerful electronics tend to drive research and development for image sensors.
Most image sensors employ charge-coupled device (CCD) or complementary metal-oxide semiconductor (CMOS) technology. Both technologies capture visible radiation and convert received radiation into electrical signals. The CCD technology is an analog device that converts incident light information into a voltage one pixel at a time, as the information is output from the CCD chip. The CMOS technology comprises an active pixel sensor array that receives light energy and complementary circuitry that converts the light energy into voltage. The voltage output can then be displayed on a video display, generally following image processing of the voltage information.
For CMOS technology, readout circuitry can be employed to transfer information from an image sensor to a video display, for instance (or to data storage, or other suitable data processor). In some cases, readout circuitry simply directs all data from the image sensor to the video display. In other cases, readout circuitry can selectively direct a subset of the image sensor data to the video display. This can facilitate fitting image sensor data to a screen size, for instance, or averaging subsets of data to mitigate image artifacts, or other examples. Dedicated hardware can be integrated into a CMOS chip to provide this selective readout capability.
Although dedicated readout circuitry can provide desirable features for data readout, it can also add some overhead and complexity. For instance, supporting multiple readout functions can involve multiple such configurations. This tends to increase design, testing and evaluation overhead. Generally, however, the benefits provided in flexibility can outweigh the overhead costs for certain applications, and selective readout hardware has become increasingly popular for optical image sensors.